The invention relates generally to paper making and, more particularly, to drying sections and methods in paper making machines.
The invention is applicable both in drying sections of the single-fabric draw type, wherein the drying fabric runs from a drying cylinder of one line to a drying cylinder of another line supporting the web in the spaces between the lines, as well as in the drying sections of the twin-wire draw type wherein the web comes into direct contact with the surfaces of the drying cylinders by means of outer drying wires and/or felts which are guided by guide rolls in the spaces between the successive drying cylinders.
In particular, a first embodiment of the invention is utilized in an arrangement, preferably in the initial drying group or groups, wherein the paper web to be dried is guided in direct contact against the surfaces of heated drying cylinders as a single-fabric draw wherein the drying fabric, which is preferably a relatively open drying wire, runs in a serpentine path from one line of drying cylinders or rolls to the other, so that the web is supported by the drying fabric in the spaces between the lines of drying cylinders. The web is placed in direct contact with the heated surface of the drying cylinders in one of the lines, while the drying wire comes into direct contact with the surface of the rolls or cylinders in the other line with the web being situated outside of the drying fabric. In this embodiment, the drying section comprises one or more successive drying groups, each consisting of two lines of cylinders and/or rolls, at least the initial drying group or groups being provided with a single-fabric draw so that the paper web is supported on the drying fabric over its entire passage through the drying group.
The drying fabric presses the web to be dried into direct contact against the surface of the drying cylinders in one of the lines while the drying fabric is in direct contact with the surface of the cylinders or rolls of the other line with the web being situated outside of the drying fabric.
A second embodiment of the invention is utilized in an arrangement wherein the web to be dried is guided into direct contact with the surfaces of heated drying cylinders and pressed against the heated cylinder surfaces by an outer drying wire and/or felt which is guided by guide rolls situated in the spaces between the successive drying cylinders. The drying section comprises one or more successive drying groups, each including two lines of drying cylinders situated one above the other, preferably in horizontal rows. In connection with the upper cylinder line, there is an upper drying wire or felt which is guided by guide rolls which are situated in the spaces between the drying cylinders. In connection with the lower cylinder line there is a second drying wire or felt which is guided by guide rolls which are situated in the spaces between the cylinders of the lower line.
Multi-cylinder drying sections of paper machines conventionally include two horizontal lines of drying cylinders, one located above the other, wherein the paper web runs in a serpentine manner between the drying cylinders of the respective lines.
A web to be dried is conventionally passed through such drying sections in either a single-wire or twin-wire draw. In a single-wire draw, the web runs from one drying cylinder in one line to the next drying cylinder in the other line, supported by the same drying wire. In twin-wire arrangements wherein upper and lower wires are used to press the web against the surfaces of the drying cylinders, the web runs from the drying cylinders of one line to the drying cylinders of the other line in an unsupported free draw. The present invention relates to drying sections and methods in which single-wire and/or twin-wire draws are used.
In drying sections of the type described above, the web is dried by coming into contact with the hot surface of a drying cylinder or cylinders. The efficiency of such contact drying depends on the nature of the contact between the web and the hot surface of the drying cylinder and it is the primary function of the drying wire or fabric to improve such contact. Another important factor contributing to the efficiency of contact drying is the temperature of the drying wire which has an important effect on the rate of drying.
Drying wires generally have a relatively open construction in order to facilitate ventilation of the space or pocket formed by the web, drying wire and the drying cylinder. It has been understood that in view of the open nature of drying wires, water vapor will pass in an uncondensed state through the wire.
A single-fabric drying section is disclosed in U.S. Pat. No. 3,503,139 assigned to Beloit Corporation. The drying section comprises two horizontal lines of steam-heated drying cylinders in which one line is situated within the wire or felt loop which therefore comes into direct contact with the cylinders while the drying cylinders of the other line are situated outside of the wire or felt loop. The drying section disclosed in this patent utilizes the customary technique of raising the temperature of the drying cylinders situated within the loop to a higher temperature than the cylinders situated outside of the wire or felt loop against the surfaces of which the web comes into direct contact. The present invention utilizes a technique which is directly contrary to the customary conventional techniques utilized in conventional multi-cylinder sections of the type disclosed disclosed in U.S. Pat. No. 3,503,139.
Conventional multi-cylinder drying sections have several drawbacks. For example, the speed of the surfaces of the drying cylinders in respective lines differ due to the different temperatures which the cylinders of the respective lines are raised and consequent differences in thermal expansion thereof. Another drawback is that the web to be dried tends to detach from the surface of the drying wire when the drying wire rather than the web comes into direct contact with the cylinder. This tendency is intensified by the positive pressures induced in the inlet nip between the drying wire and the cylinder surfaces. Detachment of the web from the drying wire can cause the web to break or form so-called bags or wrinkles in the web. Another drawback of conventional multi-cylinder dryers is that the drying wires tend to wear rather rapidly, especially in the case where the drying wires are made of plastic. This drawback is at least partly attributable to the fact that the drying wire is pressed into direct contact with the hot cylinder surface of the cylinders of one of the lines as well as to the fact that the wire is subject to a variable tension depending upon whether it is traveling in the upward or downward portions of its run.
A starting point of the present invention is an arrangement disclosed in Finnish Patent Application No. 81 2089 of Markku Lampinen of July 2, 1981, assigned to the assignee of the instant application. A multi-cylinder drying section of a paper machine is disclosed in which open drying wires are used to press the web against the cylinder surfaces and/or to support the web on the run between the cylinders. Drying wires are used which are penetrable by air whose permeability is greater than 500 m.sup.3 /m.sup.2 /h when the difference in pressure over the wire is 100 Pa. The object of this construction is to cause the water vapor to condense in the wire despite the openness of the wire when the temperature of the wire is sufficiently low. In this manner, a sort of air-conditioning in the drying section of the paper machine is accomplished so that the temperature of the drying wire, such as a wire of a single-fabric draw, is as low as possible.
The apparatus disclosed in said Finnish Application No. 81 2089 is characterized by a wire pocket formed by the drying wire to be cooled, guided by guide rolls, the pocket being brought under a negative pressure to provide an airflow through the drying wire forming the walls of the pocket to cool the drying wire. Moreover, boxes or the like provided with airflow apertures are situated on both sides of the drying wire to produce an airflow through the meshes of the drying wire.
However, it has been found that in practice it is not possible to cool the drying wire utilizing the arrangement disclosed in Finnish Application No. 81 2089 to a sufficient extent so that the desired effects can be obtained to a sufficiently high level. Thus, it appears that the drying wire tends to become warm as a result of the fact that in a single-wire draw, the drying wire is in direct contact with the heated surface of the drying cylinder on one of the cylinder lines, usually the lower cylinder line, where the paper web remains on the outer surface of the drying wire, i.e., where the drying wire itself is in direct contact with the heated surface of the drying cylinder.